Fluid pressure operated switch



Oct. 20, 1953 R. H. TIETJEN FLUID PRESSURE OPERATED SWITCH Filed Nov. 14, 1951 .1E .l l 1/ Q a 3 z %z m z a ,n w 3f, 5w

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Patented Oct. 20, 1953 UNITED STATES PATENT OFFICE FLUID PRESSURE OPERATED SWITCH Robertv H. Tietjen, Davis, Calm, assigner to the United States of America as represented bythe Secretary of the United States Air Force Application November 14, 1951, Serial No. 258,283

(Cl. 20G-81.5)

4 Claims. l

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to a fluid pressure operated switch that wil-l' operate at a lower pressure when the pressure is increasing than when the pressure is decreasing.

The primary object oi the invention is to pirovide an electric switch including two movable contacts each actuated by separate fluid pressure responsive means connected by pressure conduits to a common pressure source and wherein means is provided to retard the action of one fluid pressure responsive means in order to cause switching action in one direction at a low' pressure when the pressure is increasing and to cause switching action in the opposite direction at a higher pres-- sure when the pressure is decreasing.

Another object of the invention is to provide the pressure is increasing and to cause switching action in the opposite direction at a higher pressure when the pressure is decreasing.

A further object of thev invention is to generally improve the switching action of fluid pressure operated switches and to provide a switch that will operate at a lower pressure when the pressure is increasing than when the pressure is decreasing.

The above and other objects of the invention will become apparent upon reading the following detailed description in conjunction with theA accompanying drawing, in which:

Fig'. l is a longitudinal cross section taken through the fluid pressure operatedl switch and showing a switch which will 4close a circuit as the pressure increases and open the circuit as the pressure decreases.

Fig. 2 is a fragmentary longitudinal cross section taken through the fluid pressure operated switch and showing a switch which will open a circuit as the pressure increases and close the circuit' as the pressure decreases.

In certain control circuits it is sometimes' advantageous to have a fluid pressure operated switch responsive to a low increasing' pressure 2 to operate in one direction Land also responsive to a high decreasing pressure to operate in the other direction. It is also desirable that after operation in either direction the switch remain in the same relative position until a reversal of pressure conditions arise.

One possible and unique use for a fluid pressure operated switch of the kind described herein is in a fuel control system for a turbojet engine, wherein. there is a main fuel system and an emergency or standby fuel system. With the main fuel line connected at one point to the fluid pressure operated switch, the increasing fuel pressure during normal acceleration of the engine will operate and maint-'ain the switch in such a relative positionv as to hold the emergency fuel system in an o or inactive state. However after normal fuel pressure is attained any sudden drop in fuel pressure will operate the switch in such a direction. tobring the emergency fuel system into action and? also maintain the emergency system in operation until the normal fuel pressure is again built up and maintained.

The preferredr embodiment of the switch device isshown in Fig. 1 wherein the essential parts of the device are mounted on a base. or support I having a screw threaded projection 2 forming a fluidv pressure inlet passage 3' which sub-divides at opposite sides to form pressure lead-in branches I and 5'. Secured on the base I in the branch 4. is a pressure-sensitive bellow-s 5 having opposite end plates 'I' and 8 and enclosing a tension spring 8 attached to the end plates at its oposi'te ends. The 'spring acts to overcome the stiffness' of' the metal bellows and thus cause it to contract when the fluid' pressure therewithin diminishes. As will be obvious :any increase in Huid pressure in the bellows will cause endwise expansion thereof; Relative movement of the end plate 8 will also cause movement ofthe insulating block I0 and a metal spring arm II atta/:hed to the insulating block. Precise ladjustment of' the arm i I may be accomplished by a set-screw I2 threading through an arm I3 secured centrally of the insulating block II). A flexible lead- I4 connects the switch arm II to a terminal screw I5 in the insulating cover member I6. The latter member is secured by screws within the outer end of a casing I'I which is in turn `secured' to the base or support member I.

The other lead-in branch 5 has mounted therein a small screw threaded orifice plug I8 and a second pressure-sensitive bellows I9 including end plates 20 and 2|. A screw threaded access plug 22' inv the hase I allows for cleaning or replacing the orifice plug I8. A tension spring 23 in the bellows is attached at opposite ends to the end plates 2D and 2I as in the rst mentioned bellows Attached to the end plate ZI is an insulating block 24 and secured to the block is a metal spring arm 25. The free ends of the spring arms lI and 25 carry contact points II and 25 which in the minimum pressure condition illustrated are slightly spaced apa-rt. A flexible lead 26 connects the arm 25 with a second switch terminal 2l. Longitudinally lined up with respect to the contact points I I and 25' is an abutment screw 2S which is adjustable in the cover member IE and will cause the switch contacts to close and remain closed at a relatively high and stable fluid pressure condition and at al1 pressures over and above such stable fluid pressure.

To explain the operation of the switch in its preferred form, it may be assumed that the fitting 2 is connected to the main fuel line leading to an engine from a main fuel pump. Now as the engine is accelerated and the fuel pressure gradually increases, this line pressure affects the bellows B without any appreciable time lag and causes the switch arm il to move toward the cover member l5. The switch is now closed and closing of the circuit therethrough is adapted to actua-te a suitable relay for holding the emergency fuel system in an inactive state. Even though the fuel pressure is transmitted also into the bellows i5, the response thereof is delayed because of the throttling effect of orifice plug I8. Thus the expansion of the bellows i9 takes place at the same time that the bellows is expanding but at a slower rate of change. As the fuel pressure continues to build up to the desired maximum the switch stays closed due to the lag in movement of switch arm but finally the arm 25 contacts the abutment screw 28 and remains relatively stationary, Upon reaching a pressure level at or below the maximum, the arm I I moves out to such a position as to be bent slightly and thus maintains rm contact of the points II and 25. This continued outward movement of the bellows 5 screw 28 may be termed the overtravel of the switch arm II. Of course the bellows I9 will eventually become extended to the same extent as the bellows 5 under stable maximum fuel pressure conditions but here too the arm 25 may flex slightly as the contact 25 stops and the bellows l continues to expand. Now in case the main fuel pump fails or other failure occurs whereby the fuel pressure drops suddenly from the desired maximum operating pressure, the bellows 5 responds immediately by contracting under the influence of spring 5. After any slight overtravel has been canceled, the contact II recedes away from contact 25' to thus break the circuit and denergize the relay which was holding the emergency fuel system out of operation. The emergency fuel system now begins operation to rebuild the fuel pressure to the engine. Once the emergency system is in operation it cannot be over-ruled by the pressure responsive switch since its circuit completing relay becomes locked in by a suitable mechanical or electrical interlock.

An alternative form of the invention is shown in Fig. 2 in a fragmentary form. Here the static low pressure condition acts to maintain the switch closed instead of open, as in Fig. 1. The structural arrangement of the parts is the same as in the first or preferred form of the invention, except that the switch arms are arranged in a reafter the arm 25 has engaged abutment l lation opposite to that of Fig. 1 and the contact points carried thereon are in engagement in the zero pressure condition. The left-hand bellows includes an end plate 38 having an insulating block 40 mounted thereon. The switch arm 4I of springy strip metal is secured to the center of insulating block 40 and may be adjusted precisely by means of a set-screw 42 threaded through an arm 43 also fixed to the block 4D. A flexible lead 44 connects the switch arm 4I to the terminal 45 on the insulating cover member 46. A portion of the switch casing is designated at 41.

The right-hand bellows 4.3 includes an end plate 5I having the insulating block 54 mounted thereon and the latter carries a second spring arm 55. The adjacent ends of the metallic switch arms 4I and 55 carry contact points 4I and 55' which are in engagement when there is no fluid pressure on the pressure responsive bellows 36 and 49. A flexible lead 56 extends from the switch arm to the switch terminal screw 5l. An abutment screw `58 is adiustably mounted in the cover member 46 in a position where it will contact the switch arm 55 as the pair of bellows expands to an extent just below the maximum expansion. Thereby the open circuit condition in effect during expansion of the bellows 36 and 49 will be maintained at stable maximum pressure because of the overtravel of contact 4I. As in the first described form of the invention the left-hand bellows 35 will respond immediately to pressure change, whereas the right-hand bellows 49 will respond slowly because of the throttling effect of the orifice leading into the right-hand or slowacting bellows. Thus as the pressure in the line or container under consideration increases from a minimum the switch arm 4! will move quickly to break the circuit through the switch and as long as the pressure continues to increase up to a maximum the other switch arm 55 will lag behind the switch arm 4I. Just before the maximum pressure is reached the arm 55 will contact the abutment screw 5B and therefore even after a stable maximum pressure is reached the switch will remain open due to the overtravel of the switch arm 4I and Contact point 4I. If the pressure falls off due to failure of the pressure producing means, the bellows 3S and 49 will start to collapse but the action of the left-hand bellows 36 will be much more rapid than that of the bellows 49. Thus the switch will close very soon after the pressure starts to fall and will stay closed as the pressure continues to fall. Such a switch may be used to directly close a circuit to an electrically driven standby pump, thus acting to restore at least part of the original pressure. Of course it is obvious too that such a switch may operate a relay circuit which may in turn close a circuit to an emergency or standby pumping system.

From the above description taken in conjunction with the drawing it will be clear that I have provided a fluid pressure operated switch operable in one direction at a low pressure when the pressure is increasing and operable in the opposite direction at a high pressure when the pressure is decreasing. While there may be numerous applications or control circuits where the invention in either of its illustrated forms is useful, one particular use has been described in conjunction with an engine fuel system including a main fuel system and an emergency or standby fuel system, Several adjustments for determining the switch operating characteristics have been provided. The difference in rate of response of the pair of similar bellows may be varied by change in the orice plug leading to the slow-acting bellows. The relative spacing of the switch contact points may be adjusted by means of an adjustment screw bearing on one switch arm near its mounting point. The amount of switch contact point overtravel in the stable maximum pressure condition, that is after full expansion of the bellows, may be adjusted by means of the adjustable abutment screws 28 and 58.

The embodiments of the invention herein shown and described are to be regarded as illustrative only and it is to be understood that the invention is susceptible of variations, modifications and changes within the scope of the appended claims.

I claim:

1. In a iluid pressure system including an auxiliary source of fluid pressure, a fluid pressure operated switch comprising a housing having a fluid pressure passage extending into one side thereof and connected to two separate branch passages therein, iluid ilow constriction means in one of said branch passages, pressure responsive bellows members, each connected at one end thereof to a respective branch passage and at the other end thereof to a resilient switch arm, contact means on the respective switch arms aligned and normally in spaced relation, a tensioning means operatively associated with one of said switch arms and connected to one of said bellows members for adjusting the normal spacing between said contaot means, and an adjustable abutment means in the side of the housing cpposed to the fluid p-ressure passage and in the plane of the contacts whereby on normal uid pressure the bellows members will expand, the restricting means retardng its associated bellows member expansion and whereby the contact means will immediately close to eliminate the auxiliary source of fluid pressure and on reduction of uid pressure will immediately separate to provide an activation of the auxiliary source of fluid pressure at a point spaced from the closing point.

2. A iluid pressure operated switch comprising a housing having a fluid pressure passage extending into one end therof and operatively connected to two separate branch passages therein, bellows members, each having end plates including an apertured end plate respectively connected with said branch passages, spring members respectively connecting the opposed end plates in the respective bellows members to maintain them in contracted position upon loss of iluid pressure, a throttling means associated with one of said bellows members to retard its expansion and contraction, switch arms each connected to the respective end plates of the bellows members opposed to the apertured end plates and having contact means cooperable upon introduction or loss of iluid pressure in said housing to provide make and break to respectively activate or inactivate an auxiliary source of fluid pressure each lat different pressures.

3. The structure as set forth in claim 2 and an vadjustable abutment means in the other end of Athe housing and arranged in the plane of the switch arms to provide a limiting means to maintain said contact means in normal make or break position upon establishing a predetermined pressure condition.

4. A iiuid pressure responsive switch comprising a housing, a Iluid pressure passage extending into one end thereof and operatively connected to two separate branch passages therein, bellows members respectively connected with such branch passages at one end thereof, resilient means associated with the bellows members to bias them to a contracted position, a throttling means associated with one end of said bellows members to retard its expansion or contraction, switch arms each connected to the other ends of the bellows members respectively and having contact means cooperable upon introduction or loss of fluid pressure in said housing to provide make and break each at different pressures.

ROBERT H. TIETJEN.

References Cited in the le of this patent UNITED STATES PATENTS Number 

